The present invention relates to new blocked isocyanates based on 1,6-hexamethylene diisocyanate (HDI) and their preparation and use.
The preparation and use of blocked polyisocyanates has been known for a long time. In this procedure the free NCO groups of polyisocyanates are (temporarily) deactivated in order to arrive at products which can preferably be employed in formulations to be processed as one component (cf. D. A. Wicks and Z. W. Wicks Jr., Progr. Org. Coatings, vol. 36, 1999, p. 148 et seq. and literature cited therein). Blocked polyisocyanate hardeners based on polyisocyanates prepared from oligomeric HDI derivatives, for example, trimers (isocyanurates), have been widely introduced.
Alcohols have proved suitable as blocking agents in numerous uses (cf. also Y. Huang, G. Chu, M. Nieh and F. N. Jones, J. Coat. Techn., vol. 67, 1995, p. 33-40 and literature cited therein). However, products based on HDI derivatives are treated only on the margin in the literature reference cited above (cf. loc. cit., p. 37, line 1, this is a methanol-blocked Desmodur(copyright) N 3300 from Mobay). The use of specific high-boiling alcohols, such as 1- and 2-octanol (I. de Aguirree and J. Collot, Bull. Soc. Chim. Belg., vol. 98, 1989, 19); furfuryl alcohol (EP-A 206 071), benzyl alcohol (H. Ulrich and D. Gilmore, Urethane Chem. Appl., ACS Symp. Series # 172, ACS Washington D.C., 1981, p. 519); and cyclohexanol (EP-A 21 014) is mentioned more often.
It is a disadvantage in this connection that the blocking of HDI isocyanurate trimers with simple alcohols, such as methanol, ethanol, n-and iso-propanol etc., leads to products which have an unsatisfactory resistance to crystallization in conventional paint solvents, such as n-butyl acetate (BuAc), 2-methoxypropyl acetate (MPA), xylene (X), solvent naphtha (SN 100) etc., at room temperature (22xc2x12xc2x0 C.). After preparation of the blocked polyisocyanate hardener, a thickening of the solution is observed, sometimes with somewhat of a delay in time, resulting after a few days in complete solidification or in partial gelling of the previously clear, liquid mixture.
The tendency of alcohol-blocked HDI isocyanurate trimers to crystallize intensifies at high concentrations of the alcohol-blocked polyisocyanate in the above-mentioned paint solvents, and when the HDI isocyanurate polyisocyanate contains high contents of the xe2x80x9cideal structurexe2x80x9d of the HDI trimer, 2,4,6-tris(6-isocyanatohexyl)-2,4,6-triazine-1,3,5-trione. This indicates that the reaction products of 2,4,6-tris(6-isocyanatohexyl)-2,4,6-triazine-1,3,5-trione with three equivalents of alcohol have the greatest tendency to crystallize, while an increasing xe2x80x9ccontaminationxe2x80x9d of the latter with alcohol-blocked higher molecular weight isocyanurate polyisocyanates (pentamers, heptamers etc.) reduces the tendency of the overall mixture to crystallize. Such effects are often encountered in organic chemistry. However, it is impossible to predict the content down to which the constituents which tend to crystallize must be depleted in order to arrive at products which are stable towards crystallization.
An object of the present invention is to provide HDI polyisocyanates which are blocked with simple alcohols and which have a significantly improved resistance towards crystallization compared with the known HDI isocyanurate polyisocyanates blocked with simple alcohols, while maintaining or improving the other advantageous technology properties of blocked HDI isocyanurate polyisocyanates.
Surprisingly, it has been possible to achieve this object by using HDI polyisocyanates which contain a portion of iminooxadiazinedione groups (asymmetric trimers, AST; HDI-AST polyisocyanates).
The present invention relates to blocked polyisocyanates which are the reaction product of polyisocyanate resins prepared from HDI and containing iminooxadiazinedione groups in an amount of at least 20 mol %, based on the total moles of iminooxadiazinedione and isocyanurate groups, with alcohols of formula (I)
ROHxe2x80x83xe2x80x83(I),
wherein
R represents aliphatic, straight chain or branched, optionally cyclic, optionally substituted (Oxe2x80x94R1, NR12), C1-C6 radicals, preferably C1-C4 radicals, and
R1 has the same meaning as R.
The present invention also relates to a process for the preparation of blocked polyisocyanates by reacting polyisocyanate resins prepared from HDI and containing iminooxadiazinedione groups in an amount of at least 20 mol %, preferably at least 30 mol %, based on the total moles of iminooxadiazinedione and isocyanurate groups, with alcohols of formula (I).
The present invention finally relates to the use of the blocked HDI polyisocyanates according to the invention and containing iminooxadiazinedione groups for the preparation of polyurethane plastics and coatings, preferably for coating substrates.
The preparation of HDI-AST polyisocyanates is described, for example, in EP-A 798 299. The use of HDI-AST polyisocyanates for the preparation of blocked polyisocyanates is disclosed in EP-A 947 531, p. 3, I. 23-25. However, EP-A 947 531 provides polyisocyanates which are blocked with malonic esters and are stabilized with formaldehyde against thermal yellowing. Alcohol-blocked polyisocyanates are not dealt with in EP-A 947 531. Furthermore, polyisocyanates which are based exclusively on HDI are not preferred as starting components for the preparation of the blocked polyisocyanate claimed in EP-A 947 531, as mentioned explicitly in loc. cit. p. 3, I. 26-28.
The previous publications of the prior art lack any indication that the combination of xe2x80x9calcohol+HDI-AST polyisocyanatesxe2x80x9d could lead to products with advantageous properties. The expert cannot deduce generally from the previous publications of the prior art that the resistance of blocked HDI-AST polyisocyanates towards crystallization differs in any manner from their counterparts based purely on isocyanurate (symmetric trimers). Only the generally lower viscosity of AST-PIC compared with their symmetric counterparts is known from the literature (Proc. XXIVth Fatipec Conference, Jun. 8-11, 1998, Interlaken, Switzerland, vol. D. pp. 131-145). There is no xe2x80x9clower viscosity of the non-blocked hardenerxe2x80x94better stability of the blocked form to crystallizationxe2x80x9d connection. It is generally known that with increasing xe2x80x9ccontaminationxe2x80x9d of organic substances a lowering in melting point as a rule occurs, but whether and how long such mixtures are stable towards crystallization is completely uncertain and always requires experimental trials in the individual case.
The blocked HDI-AST polyisocyanates according to the invention can be prepared in an industrially simple manner by reaction of the HDI-AST polyisocyanates with alcohols. Pure alcohols or mixtures can be used in this reaction. Suitable alcohols for the blocking reaction are all, optionally substituted (xe2x80x94Oxe2x80x94R1, xe2x80x94NR12; R1=straight chain or branched C1-C4-alkyl), linear, branched, or cycloaliphatic C1-C6 alcohols. Examples of said alcohols are methanol, ethanol, methoxyethanol (ethylene glycol monomethyl ether), ethoxyethanol (ethylene glycol monoethyl ether), dimethylaminoethanol, diethylaminoethanol, propanol (any isomer), methoxy propanol (any isomer), ethoxy propanol (any isomer), dimethylaminopropanol (any isomer), diethylaminopropanol (any isomer), butanol (any isomer), pentanol (any isomer), and hexanol (any isomer). Preferred alcohols are methanol, ethanol, n- and iso-propanol and methoxypropanol isomers.
It is irrelevant whether or not the alcohols used as the blocking agent are employed in exactly the stoichiometric ratio, based on the free NCO groups of the HDI-AST polyisocyanates. A small excess of alcohol is usually employed: however, a certain deficit of alcohol is also tolerated, since the free NCO groups of the polyisocyanates can react to a small extent beyond the pure NCOxe2x80x94OH reaction in side reactions, e.g. to form allophanates. The NCOxe2x80x94OH molar ratio in the blocking reaction is preferably between 1.2 and 0.8, more preferably between 1.1 and 0.9.
To adapt the properties of the products according to the invention to the particular application requirements (increase in the crosslinking density by increasing the average blocked NCO functionality in the finished hardener, influencing of the elasticity by components which impart hardness/softness etc.), a so-called prelengthening can take place in addition to the blocking reaction. For this, less than the stoichiometric amounts (based on the number of NCO groups in the starting resin) of alcohols, amines and/or amino alcohols are added to the polyisocyanate component containing free isocyanate groups, i.e., the above-mentioned prelengthening agents containing at least 2 OH or NH groups per molecule.
Compounds which are particularly suitable for this are optionally branched diols and triols which optionally contain cycloaliphatic segments and preferably contain an average of 3 to 30 carbon atoms, in which the carbon chain can optionally be interrupted by functional groups, in particular ether and ester groups. Compounds which are preferably employed are C3-C12 di- and triols, such as Isomers and mixtures of the following compounds: propanediols, butanediols, pentanediols, hexanediols, heptanediols, octanediols, nonanediols, decanediols and trimethylolethane and -propane. The NCO groups which have not been consumed in the xe2x80x9cprelengtheningxe2x80x9d reaction are subsequently or simultaneously reacted with the actual blocking agent. The amount of NCO groups reacted in the xe2x80x9cprelengtheningxe2x80x9d reaction, based on the total amount of NCO groups of the starting resin, is 1 to 50%, preferably 5 to 20%.
The blocking and also the prelengthening reaction can be accelerated by using known catalysts from polyurethane chemistry. More catalyst than is absolutely necessary to accelerate the NCOxe2x80x94OH reaction to an industrially advantageous levelxe2x80x94i.e. as a rule 2 to 24 hours until the absence of free NCO groups in the reaction mixturexe2x80x94can also be employed here, since the reaction of the blocked HDI-AST polyisocyanates according to the invention with polyol components for the preparation of polyurethane plastics and coatings is as a rule also catalyzed.
Examples of suitable catalysts described in D. A. Wicks and Z. W. Wicks Jr., Progr. Org. Coatings, vol. 36, 1999, pp. 148 and literature cited therein. These include tin compounds of the RnSnRxe2x80x2(4xe2x88x92n) type, in pure form or as any desired mixtures of different species with one another, optionally also with a proportion of tin-free species, wherein
R represents identical or different, optionally branched radicals from the C1-C12-alkyl series, preferably n-butyl and n-octyl,
n represents 1, 2 or 3 and
Rxe2x80x2 represents:
xe2x86x92C1-C20 alkanoate, xe2x80x94Oxe2x80x94C(O)xe2x80x94Rxe2x80x3, preferably acetate, 2-ethylhexylhexanoate and laurate, or
xe2x86x92C1-C20 thioglycollate, xe2x80x94Sxe2x80x94CH2C(O)xe2x80x94ORxe2x80x2xe2x80x3, and
Rxe2x80x2xe2x80x3 preferably represents n-butyl and iso-octyl.
The blocked HDI-AST polyisocyanates according to the invention can be employed for a large number of uses. Examples of these are general industrial painting, automobile initial painting (xe2x80x9cOEMxe2x80x9d)xe2x80x94both the filler and the clear coating sectorxe2x80x94coating of glass and coil and can coating. The blocked HDI-AST polyisocyanates according to the invention are preferably employed in coil and can coating. In addition to the good stability of the blocked HDI-AST polyisocyanates according to the invention to crystallization, the good results in the T-bend test on metal sheets coated in this way, the very good subsequent tear strength of the metal sheets deformed in the T-bend test after exposure to heat and a very good resistance to over-stoving are other advantages (for details, cf. use examples 9 and 10).
Since the blocked HDI-AST polyisocyanates according to the invention are very highly viscous products in the pure, undiluted form, solvents are preferably added for better handling. To keep the content of volatile organic compounds (VOC) as low as possible, however, it is desirable to choose a solvent content which is as low as possible, but sufficient to establish a low viscosity necessary for good processability. The general property of asymmetric diisocyanate trimers of having a significantly lower viscosity than their symmetric isomers is an additional benefit. As a result the solvent content of blocked HDI-AST polyisocyanates can be reduced for the same viscosity and NCO functionality of the hardener compared with systems based purely on isocyanurate. Lower viscosities for the same solvent content in the paint mixture result.
Possible solvents are those known from the paint industry. Examples include hydrocarbons (preferably aromatic), esters, ketones, ether-esters, alcohols and any mixtures thereof.
The blocked HDI-AST polyisocyanates according to the invention can be combined with known polyols (xe2x80x9cbindersxe2x80x9d). Examples include polyester polyols, polyacrylate polyols, polyurethane polyols, epoxy resins, amino resins (melamine resins, urea resins and benzoguanamine resins) and mixtures thereof. The equivalent ratio of binder to blocked hardener is preferably selected such an equivalent ratio of 1:1 results. However, the product can also be over-crosslinked (blocked NCO:OH greater than 1) and to a certain extent under-crosslinked (blocked NCO:OH less than 1).
The blocked HDI-AST polyisocyanates according to the invention are suitable for coating a large number of materials, such as glass, concrete and metal. They are preferably employed for painting sheet metals (in so-called xe2x80x9ccoilxe2x80x9d and xe2x80x9ccan coatingxe2x80x9d).